A Comparison of the Microstructural Feature and Bonding Strength of Plasma-Sprayed Hydroxyapatite Coatings with Hydrothermal and Vacuum Post-Heat Treatment

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Abstract
Plasma-sprayed hydroxyapatite (Ca10(PO4)6(OH)2, HA) is an attractive biomaterials because it can provide the osteoconductivity and osseointegration to form a chemical bonding to bone. In order to improve the properties of HA, the post-heat treatment was performed and the effect of an ambient atmosphere on phase purity, crystallinity and bonding strength of plasma-sprayed HA coatings (HACs) was investigated. On the basis of quantitative analysis in crystallinity and phase content, autoclaving hydrothermal treatment was found to be effective for the elimination of impurity phases and amorphous calcium phosphate. It should be noted that plasma-sprayed HACs with a crystallinity of 84.5% and very low impurity phase content of 1.4 mass% can be obtained through a low temperature (200°C) hydrothermal treatment. Experimental evidence confirmed that the saturated steam atmosphere plays an important role in lowering the heating temperatures and promoting HA crystallization. Furthermore, hydrothermal-treated HACs show superiority in prominent OH− and PO43− groups than vacuum heating HACs. In addition, the bonding strength of the coating layer also can be improved from 35.7 MPa (vacuum heating) to 39.4 MPa (150°C hydrothermal treatment). From the evaluation of microstructural features and properties, an excessively high heating temperature will cause contraction with HA crystallization resulting in detrimental large cracks that lead to bonding degradation. Consequently, hydrothermal treatment seems very promising as a way of improving the properties of HA coatings.